1. Field of the Invention
The invention relates to narrow band lasers and particularly to an excimer or molecular fluorine laser having output coupling interferometer.
2. Discussion of the Related Art
Narrow band excimer lasers (xcex=193 nm, 248 nm) are applied in photolithographic applications for production of integrated circuits. Excimer laser radiation is used for making structures in the dimensional range of  less than 0.18-0.25 xcexcm (KrF-laser radiation) or  less than 0.13-0.18 xcexcm (ArF-laser radiation). The molecular fluorine laser emitting around 157 nm (F2-laser) is being developed for feature sizes  less than 0.13 xcexcm. Achromatic imaging optics are difficult to produce for this wavelength region. For this reason radiation of narrow bandwidth is desired to control imaging errors caused by chromatic aberration. Acceptable bandwidths are typically less than 0.6 pm.
Another important beam parameter is the spectral purity, or the bandwidth which contains 95% of the output pulse energy. High numerical aperture (NA) optics use  less than 1 pm bandwidth radiation. This can achieved by using of two spectral narrowing elements such as a grating and intracavity etalon or etalon output coupler.
Etalon outcoupling mirrors have been used for a long time and in various different types of lasers. A simple example of a plane-plane cavity for an excimer laser can be formed by a highly reflective (HR) back-mirror and an uncoated solid etalon as an outcoupling resonator reflector.
U.S. Pat. Nos. 5,901,163 and 5,856,991 each to Ershov relate to a resonator including an etalon output coupler for a narrow band excimer laser, as shown in FIG. 1 (which is FIG. 3 of the ""991 patent). The resonator consists of a line narrowing module (18) consisting of an echelle grating and a prism beam expander, and a plane-parallel air spaced etalon (44) as an outcoupling mirror.
The echelle grating based line narrowing module produces a laser beam having a spatial variation in wavelength (chirp) along a beam cross section direction (direction of dispersion). FIG. 2 shows a typical spatial distribution of a laser spectrum across the beam created by the grating. The laser resonator used for generating the spectrum in FIG. 2 consists of an echelle grating, prism beam expander and a typical partially reflecting outcoupling mirror having a reflectivity of, e.g., 20-25%.
Thus, for the arrangement of FIG. 1, the line narrowing module (18) provides a spatial distribution of wavelengths at the outcoupling etalon that is approximately given by:
xcex(x)=xcex(0)+(dxcex/dx)xxe2x80x83xe2x80x83(equation 1);
where x is the coordinate along the short beam axis, and x=0 is the beam center. For the example depicted in FIG. 2, the xe2x80x9cspatial chirpxe2x80x9d is dxcex/dx≈0.83 pm/mm. This value depends on the linear dispersion of the echelle grating and the laser design (i.e., the distance between the grating and outcoupling etalon, the discharge width, etc.).
FIGS. 3a, 3b show two calculated spatial distributions of laser spectra for two different gratings (dxcex/dx=0.83 pm/mm and 1.24 pm/mm), an airspaced plane-parallel uncoated etalon with FSR=1.6 pm as outcoupler and otherwise the same resonator designs. FIG. 3c shows the measured spectrum for a grating with dxcex/dx=1.24 pm/mm and an outcoupler etalon with FSR=1.6 pm. The calculations are in a good agreement with the experimental findings (i.e., compare FIGS. 3b and 3c).
To avoid xe2x80x9cside modesxe2x80x9d the following relation is fulfilled:
(dxcex/dx)xc2x7bxe2x89xa60.5 FSRxe2x80x83xe2x80x83(equation 2);
where b is the beam width in front of the etalon. Higher values for dxcex/dx can be achieved by using more highly dispersive gratings, or bending the grating such as is disclosed in U.S. Pat. No. 5,095,492 to Sandstrom. As it is desired to produce still smaller structures on silicon substrates, it is desired to further reduce the spectral purity of excimer laser exposure beams.
It is therefore an object of the invention to provide a narrow band excimer or molecular fluorine laser having improved spectral purity.
In accordance with this object, an excimer or molecular fluorine laser is provided including a discharge chamber filled with a gas mixture, multiple electrodes within the discharge chamber connected to a power supply circuit for energizing the gas mixture, and a resonator including the discharge chamber and a pair of resonator reflectors for generating an output laser beam.
In a preferred embodiment, the resonator includes an interferometer including a pair of opposing reflecting surfaces configured so that the interferometer is tuned to have a response maximum at a selected wavelength around approximately an intensity maximum of the beam incident upon the interferometer, and at least one sideband of the response spectrum of the interferometer is also within the intensity spectrum of the beam incident upon the interferometer. At least a first of the pair of opposing reflecting surfaces is configured such that the first and second opposing reflecting surfaces have a varying optical distance therebetween over the incident beam cross-section which serves to suppress the at least one sideband around the response maximum to improve spectral purity such that the interferometer with non-parallel opposing reflecting surfaces has a response spectrum including a narrow response maximum and at least one sideband which is substantially suppressed to provide an output beam at high spectral purity.
In a preferred embodiment, one of the resonator reflectors is an output coupling interferometer wherein the pair of opposing reflecting surfaces is tuned to produce a reflectivity maximum at a selected wavelength for narrowing a linewidth of the output laser beam. Alternatively, the interferometer may be configured to operate in transmissive mode within the resonator.
Preferably, at least the first reflecting surface is non-planar, and may include a step, a recess or a raised or recessed curved portion of a quarter wavelength in height or depth, respectively. Also preferably, at least the first reflecting surface has a curvature such that the first and second opposing reflecting surfaces have a varying optical distance therebetween over an incident beam cross-section that serves to suppress outer portions of the reflectivity maximum to improve spectral purity. The first surface may have a constant curvature over at least an incident beam width. The first surface may be part of a coupling component coupled to a base optical block opposing the second surface of the pair of opposing reflecting surfaces.
A beam expander may be disposed before the output coupling interferometer. The beam expander reduces the divergence of the beam incident at the interferometer, the resolution of the interferometer is improved, and the spectral purity is improved in accord with the object of the invention. The beam expander may include one or more beam expanding prisms or a lens arrangement.